The field of the present invention is vibratory separation of dry, fine material.
Vibratory separators have long been used for the separation of both dry and wet materials. Such separators may be circular or rectangular in cross section. They generally include housings which are mounted resiliently and which include a vibration generating device. Screens are fixed to the vibratory housings in substantially horizontal orientation such that material fed to the vibrating screens will be properly screened. Various vibratory motions may be employed to work the material on the screen in the most advantageous manner. Frequently discharge openings are provided both above the screening mechanism and below for retrieving the separated materials.
A variety of devices have been employed to enhance separation using such devices. One such mechanism is to employ two screens in parallel. A finer screen is placed above a courser screen or perforated plate. Sliders are positioned between the screens. The sliders have a flat surface to ride on the lower screen or perforated plate and extend upwardly to adjacent the upper screen. The sliders provide some momentum when banging against the upper screen and provide some sheering action on material hanging through the screen. These devices are often referred to as self-cleaning screen assemblies. The object of such device is to provide enhanced screening on the upper screen with minimum blinding. The lower screen or perforated plate is designed to hold the sliders in position.
Another mechanism which has been employed to enhance screening is to provide brushes having either bristles or rubber flaps extend to the surface of the upper fine mesh screen. The brushes move about the upper surface to help prevent blinding of the screen.
The screening of fine grained or powder-like materials in the dry state has been a substantial source of problems with vibratory screening devices. The terminal velocity of very small particles is so low in atmosphere that it is very difficult to screen the material. Forces between particles also come to interfere with the separation and screening process. As an example, alumina powder made up of 100 micron size particles can be sifted through a 150 micron mesh vibrating screen at 10 times the rate of alumina powder made of 1 micro size particles through the same mesh size.